html,
body {
  width: 100%;
  height: 100%;
  padding: 0;
  margin: 0;
}
.content-box {
  height: 100%;
  position: relative;
  padding: 100px 0;
  box-sizing: border-box;
}
/* 上 */
.t-box {
  position: absolute;
  top: 0;
  height: 100px;
  width: 100%;
}
/* 中 */
.m-box {
  height: 100%;
  background: #ccc;
}
/* 下 */
.b-box {
  position: absolute;
  bottom: 0;
  height: 100px;
  width: 100%;
}
